Fish Alarm System Cracked: "Schreckstoff" Mystery Solved After 86 Years

Researchers in Japan have finally unraveled the long-standing mystery of how fish communicate danger to each other. The culprit, it turns out, is not a single substance but a two-part chemical message.

This fishy whodunit dates back to 1938 when Nobel laureate Karl von Frisch observed minnows acting spooked when an injured shoal mate was introduced to their tank. He theorized that the injured fish released a fear-inducing substance, which he called "schreckstoff" (German for "frightening stuff"), that their kin detected through smell.

Scientists later confirmed this alarm system in many freshwater fish species, but the identity of schreckstoff remained elusive for decades. Numerous attempts to isolate the substance using skin extracts proved unreliable.

The breakthrough came from a shift in focus. Led by Yoshihiro Yoshihara at the RIKEN Center for Brain Science, the team bypassed traditional methods and instead examined brain activity. They monitored zebrafish brains as they encountered water containing damaged skin from zebrafish, goldfish (same superorder), and medaka (different superorder).

Their findings revealed a fascinating twist. Three distinct brain regions reacted to the damaged skin, each with its own specificity. One responded to all types of skin, another only to zebrafish and goldfish (Ostariophysi superorder), and the last solely to zebrafish.

This selectivity suggested a two-part message system. The researchers hypothesized that one molecule signaled danger ("danger!") while another identified the source ("one of us!").

Using advanced techniques, they isolated and named the two molecules: Daniol sulfate, specific to zebrafish, and Ostariopterin, found across the Ostariophysi superorder. Notably, zebrafish skin contained both molecules, while goldfish skin only had Ostariopterin.

The team's hunch was validated when zebrafish exposed to both Daniol sulfate and Ostariopterin displayed a strong fear response. This reaction, along with activity in the amygdala (fear center) of the brain, only occurred when both molecules were present.

This two-factor communication system has broader implications. "Even humans react to odors produced by others," says Yoshihara. This study suggests a potential common thread – a biological code using dual messages for social communication through smell.

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